The oil and gas products that are contained, for example, in sandstone earth formations, occupy pore spaces in the rock. The pore spaces are interconnected and have a certain permeability, which is a measure of the ability of the rock to transmit fluid flow. When some damage has been done to the formation material immediately surrounding the bore hole during the drilling process or if permeability is low, a hydraulic fracturing operation can be performed to increase the production from the well.
Hydraulic fracturing is a process by which a fluid under high pressure is injected into the formation to split the rock and create fractures that penetrate deeply into the formation. These fractures create flow channels to improve the near term productivity of the well. After the parting pressure is released, it has become conventional practice to use propping agents of various kinds, chemical or physical, to hold the crack open and to prevent the healing of the fractures.
The success or failure of a hydraulic fracture treatment often depends on the quality of the candidate well selected for the treatment. Choosing an excellent candidate for stimulation often ensures success, while choosing a poor candidate normally results in economic failure. To select the best candidate for stimulation, there are many parameters to be considered. The most critical parameters for hydraulic fracturing are formation permeability, the in-situ stress distribution, reservoir fluid viscosity, skin factor and reservoir pressure.
During an original completion, oil or gas wells often contain layers bypassed either intentionally or inadvertently. Subsequent restimulation programs designed to identify underperforming wells and recomplete bypassed layers have been unsuccessful partly because the programs tend to oversimplify a complex multilayer problem and focus on commingled well performance and well restimulation potential without thoroughly investigating layer properties and layer recompletion potential. The complexity of a multilayer environment increases as the number of layers with different properties increases. Layers with different pore pressure, fracture pressure, and permeability can coexist in the same group of layers. The biggest detriment for investigating layer properties is a lack of cost-effective diagnostics for determining layer permeability, pressure, and quantifying the effectiveness of previous stimulation treatment(s).
Conventional pressure-transient testing, which includes drawdown, buildup, injection, or pressure-falloff testing, can be used to identify an existing fracture retaining residual width from a previous fracture treatment(s), but conventional testing requires days of production and pressure monitoring for each single layer. Consequently, in a wellbore containing multiple productive layers, weeks to months of isolated-layer testing can be required to evaluate all layers. For many wells, the potential return does not justify this type of investment.
Alternative methods, like an annulus-injection test for diagnosing an open fracture from a previous stimulation treatment(s) are, at best, qualitative and the interpretation is subjective. This method is described in detail in a paper entitled “Screening Restimulation Candidates in the Antrim Shale”, SPE 29712 presented by Hopkins, C. W. et al, at the 8-10 Nov. 1994 SPE Eastern Regional Conference and Exhibition, Charleston, W.Va. The annulus-injection test, which was initially developed to identify restimulation candidates in low-permeability gas reservoirs, requires slowly injecting water into a partially depleted formation until the wellbore or wellbore and fracture fill with water. “Fillup” is determined by a rapid increase in pressure, and pressure is always maintained below the fracture pressure of the formation. A well that fills quickly is considered unstimulated, and a well that fills slowly is presumed to have a high-conductivity fracture in communication with the wellbore. Anticipated fillup volumes can be calculated from the proppant volume pumped in a previous stimulation treatment(s), but actual fillup volumes can differ from theoretical fillup volumes substantially; thus the annulus-injection test may not yield accurate data.
Also known in the prior art are various methods that include radioactive logging. One of these methods is also described in a paper “Measuring Hydraulic Fracture Width Behind Casing Using a Radioactive Proppant”, SPE 31105 presented by Reis, J. C. et al, at the 14-15 Feb. 1996 SPE Formation Damage Control Symposium, Lafayette, Louisiana. Gamma ray or spectral gamma ray logging devices can be used to identify an open near-wellbore fracture provided previous stimulation treatments were tagged with radioactive isotopes and provided the radioactivity can be measured; however, the depth of investigation of the radioactive logging tools is restricted to within a few inches or feet of the wellbore.
Two other tests that can be used to diagnose a fracture retaining residual width are impulse and slug tests. Impulse tests require an injection or withdrawal of a volume of fluid over a relatively short time period followed by an extended shut-in period, and slug tests require an “instantaneous” imposition of a pressure difference between the wellbore and the reservoir and an extended shut-in period. Typically, the instantaneous pressure difference is created by placing a “slug” of water or a solid cylinder of known volume into the wellbore. The primary difference between the tests is the time of injection or production and both can be used to identify a fracture retaining residual width. However, like the annulus-injection test, the pressure during impulse or slug test is maintained below the fracture pressure of the formation. Similar to conventional pressure-transient testing, an impulse or slug test designed to determine the presence of a fracture retaining residual width from a previous fracture treatment(s) can require relatively long periods of pressure monitoring.
What is needed is an improved technique that can identify objectively and rapidly the presence of a pre-existing open fracture from a previous stimulation treatment(s) and that is economically attractive.